1881.] on Magnetic Disturbance, Aurorce, and Earth Currents. 667 



strengths of the actual disturbing forces at the two places can only 

 be obtained by comparison of the scale values at the two places. 



I will draw your attention to one other point on this day. At 9 p.m. 

 the disturbances are all in the same direction, but about 11 p.m., 

 whilst St. Petersburg agrees in direction with the others in a very 

 violent phase of the storm, at Toronto the direction of the deflections 

 is reversed, and this reversal of curves continues until about the end 

 of this first of the three storms. 



The second storm, beginning about 11.30 a.m. on the 12th and 

 lasting until the next morning, was the most remarkable of the three. 

 It not only baffles the telegraph clerks, who wish to keep out earth 

 currents from their lines, but it even goes beyond the powers of the 

 magnetic observatories which are specially designed to watch over 

 them. Thus at Toronto the line goes off the edge of the paper on 

 which the photographic record is taken. At Melbourne the motion is 

 BO rapid, and also at Vienna, that the j^late is not sensitive enough to 

 receive the impressions, the motion is too quick even for iDhotography. 

 At the time of greatest disturbance, about 12.20 mid-day, it is very 

 remarkable that at Lisbon and at Zi-ki-Wei, near Shanghai, in China, 

 two places nearly in the same latitude but nearly 9 hours apart in 

 time, the vertical force is increased in precisely the same way and to 

 the same amount at the same instant. 



At Zi-ka-Wei, in China, the sudden change in the horizontal force 

 on the needle amounted to about one hundredth part of the total 

 horizontal force, and at St. Petersburg the change in the horizontal 

 force amounted to one thirty-fifth part of the horizontal force, and the 

 total force was changed by about one eightieth part of its full value. 



These magnetic changes are so large as to be quite comparable, 

 as we see, with the earth's total force, so that any cause which is 

 shown to be incompetent from the nature of things to produce the one 

 can hardly be held to account for the other. 



Since, as I have shown, the large disturbances and the small 

 disturbances do not follow totally different laws but agree equally 

 well all over the earth, in so far as they agree we must attribute them 

 to the same cause. 



During this August storm, as also during the remarkable storm of 

 January 31st last, great difficulties were experienced in working the 

 telegraph lines, and Mr. Preece has been kind enough to send me 

 particulars of these storms. 



I am also greatly indebted to the Astronomer Royal for sending 

 me tracings of the earth current photographic records taken at Green- 

 wich Observatory during the August storm on two separate wires, 

 one running from the north-east and the other from the south-east to 

 Greenwich. The two tracings are bent opposite ways at the same 

 time, so that when a current was running on one line towards Green- 

 wich, on the other it was running away from it, and comparing 

 these curves with the earth - current records from Derby and 

 Haverfordwest and other places, it appears that the general direction 



